CMOS image sensors are widely used in many modern day electronic devices to convert an optical image to an electric signal. A CMOS image sensor can be fabricated as a back-side illuminated or as a front-side illuminated CMOS image sensor. Back-side illuminated CMOS image sensors, which receive light on a back-side of a workpiece, offer superior performance (e.g., high resolution) over front-side illuminated CMOS image sensors.
Light incident on a back-side of a thinned semiconductor substrate, travels through a thin layer of semiconductor material to a plurality of photodetectors located within a front-side of the semiconductor substrate. The incident light generates carriers within the photodetectors which are transferred to back-end metal wiring levels that provide the carriers as an electric signal to a signal processing unit.